The configuration of powered underwater vehicles has evolved through many years based on certain understood hydrodynamic and mechanical requirements. Aerodynamic considerations have resulted in somewhat similar shapes for lighter-than-air vehicles such as dirigibles and blimps. Where significant velocity through the fluid medium is required, the art seems to have settled on a generally tubular shape, rounded at the front and tapering toward the rear with the diameter made as small as the internal mechanism and/or flotation requirements will permit to minimize frontal area. This general configuration has been evident in the usual configuration of airships, of manned submarine vehicles and of unmanned vehicles such as torpedoes. The power required to drive such a vehicle through the fluid medium varies with factors such as the effective frontal area, skin friction, and drag caused by separation of the flow over the surface of the body resulting in turbulence. A conventional way of avoiding flow separation over the rearward surfaces of such vehicles is to provide a tapering surface free of abrupt discontinuities with a propeller or impeller at or toward the rear.
Because of certain obvious advantages, some efforts have been made to fabricate and test experimental vehicles of spherical configuration. Such vehicles have inherently greater internal volume relative to their surface area than other shapes, and they have greater resistance to external pressure so can be lighter than conventional shapes because of less need for internal bracing or ribs. With greater diameter and less internal bracing required, a spherical vehicle could accommodate larger objects within than a tubular vehicle of comparable cubic content. Where a sonar must be incorporated, the larger diameter permits the use of a transducer array of much greater area than can be accommodated at the front of a tubular vehicle, so much better sonar performance could be realized.
Despite the above and possible other advantages of a spherical body for underwater vehicles, they have not been used in the past because testing has indicated that such bodies are inherently unstable. Generally spherical lighter-than air vehicles have been used as balloons, but not as dirigibles or blimps, probably because the frontal area appeared excessive. When attempts were made in the past to move a spherical body through the water at any significant velocity, the boundary layer flow in the aft part of the sphere became separated at first one radial position and then another. This results in a low pressure at the separation region while high pressures act elsewhere, causing the sphere to be slowed toward the low pressure region. This displacement results in slowing of flow in said first region which causes the flow to again become attached there but to become detached elsewhere. The sphere will then move toward the new low pressure region. This phenomenon applicable to both air and water vehicles will continue causing the vehicle to tend to oscillate back and forth. Not only is the oscillation unacceptable, but the drag becomes prohibitive and so also does power consumption. At the present time a further disadvantage is that all sorts of existing storage and mooring facilities, from hangars to harbor berths to torpedo tubes, are designed to accommodate the above described tubular shaped vehicles.